(a) Field of the Invention
This invention relates to the synthesis of urea and more particularly to an improved method for maintaining the temperature in a urea synthesis autoclave at a constant level.
(B) Prior Art
Urea is used extensively as a fertilizer and in fertilizer formulations. Commercial synthesis of urea is from dry carbon dioxide and ammonia. Under temperature and pressure, ammonia adds to one of the double bonds of the carbon dioxide to yield carbamic acid, which reacts with a second molecule of ammonium to form ammonium carbamate. The generalized reversible equation is: ##STR1## Under pressure and temperature the ammonium carbamate simultaneously converts to urea and water. An excess of ammonia is present. The generalized reversible equation is: ##STR2##
The urea synthesis effluent leaving a urea synthesis autoclave contains urea, water and unreacted ammonium carbamate and excess ammonia, if used. The urea synthesis effluent is subjected to pressure distillation to separate therefrom the unreacted ammonium carbamate (plus excess ammonia) in the form of a gaseous mixture of ammonia and carbon dioxide. The gaseous mixture is absorbed in an absorbent such as water and an aqueous solution of urea to form a recycle solution to be recirculated into the urea synthesis autoclave. In prior art processes, the temperature of the ammonia and recycle solution fed into the urea synthesis autoclave tends to be higher with time as the process of recovering unreacted ammonia and carbon dioxide progesses. As it is desirable to have the synthesis autoclave as a vessel for maintaining the dwelling time, temperature and pressure at any suitable level in order to attain any desired conversion ratio, the relentless increase with time of the quantity of heat in the synthesis autoclave becomes excessive and undesirable. As a result partly gasified ammonia and carbon dioxide leaves the synthesis autoclave, and the conversion ratio consequently falls. The problem is to remove the excess heat of reaction by suitable means whereby the amount of the gaseous ammonia and carbon dioxide leaving the synthesis autoclave is reduced and the decrease of the conversion ratio is avoided. The normal method of removing such excess heat of reaction is to place a heat-recovery device in front of the urea synthesis autoclave. In that method the outlet temperature of the synthesis autoclave is controlled by controlling the recovered heat quantity. In that case the temperature of the heat recovering device is reduced. Consequently, a larger heat-transfer area is required in order to obtain the same steam generation. Also, the urea producing velocity is reduced, which means that the urea synthesis ratio becomes undersirably small.
In two-stage synthesis of urea the temperature of the second stage is often controlled by introducing a part of the carbon dioxide and all of the ammonia into the first stage and introducing the rest of the carbon dioxide into the second stage as disclosed in U.S. Pat. No. 3,105,093, particularily the prior art section thereof. Temperature control is achieved by varying the amount of carbon dioxide introduced into the second stage. However, that method of controlling the temperature has very poor adjustment sensitivity. Also, the opening of the controlling valve used to adjust the carbon dioxide quickly clogs.
Rothkrans discloses in the specification of U.S. Pat. No. 3,105,093 a method for controlling the temperature in the urea synthesis zone, where all of the CO.sub.2 and NH.sub.3 is introduced into the heat-exchange zone, and the amount of heat removed in the heat-exchange zone is regulated by continuously controlling the pressure of steam generated in the heat-exchange zone in response to the temperature of the urea synthesis zone. However this method involves the disadvantages that the temperature and pressure of the steam generated upon heat recovery in the heat-exchange zone varies with the temperature control, and that such variation is unfavorable when the steam is used for other heating purposes.
The known methods of controlling the temperature in the urea synthesis zone suffer from a number of defects and do not actually achieve such temperature control.